1. Field of the Invention
The present invention involves the dental specialty of orthodontics and in particular, an improved instrument used to form orthodontic wires.
2. Statement of the Problem
In the broadest sense, the practice of orthodontics can be reduced to the steps of directing carefully regulated corrective forces onto individual teeth. Such forces are typically generated by various types of energy-storing springs that initiate and then maintain the physiological processes supporting tooth movement. The current orthodontic armamentarium includes many configurations of such energy-storing springs, many of which have been developed for specific but common challenges facing orthodontists. The most common energy-storing spring used in nearly all orthodontic treatment methodologies is the archwire. Archwires are formed in the familiar U-shape of the human dental arch. Such wires normally extend around the front of the arch from molar to molar. Archwires provide the prime tooth-moving motive force and provide the continuity of forces around the arch. Archwires generate forces that span multiple teeth such as leveling, arch development (expansion) and transverse expansion, and have the capability of anchoring all of the teeth of one arch in apposition to the other arch for anterior-posterior correction of the entire occlusion.
In practice, devices known as orthodontic brackets are rigidly attached to each tooth of an arch. Brackets feature a horizontally oriented and labially or buccally opening slot that serves to accept and engage an archwire. Once the archwire is inserted into a bracket's arch slot, it is retained therein by ligatures that engage tie-wings of the bracket. U.S. Pat. No. 3,504,438 to Wittman et al. discloses orthodontic brackets with arch slots and an archwire in place.
Arch slots are defined by two parallel slot walls and a slot floor that is oriented perpendicular to the walls. Within the orthodontic profession, standards for arch slot dimensions have been established. According to those standards, an arch slot width may be about 0.018 inches or 0.022 inches wide and about 0.028 or 0.030 in. deep. As can be appreciated then, the arch slot in cross-section can be thought of as being rectangular, but with one side of the rectangle open to allow the archwire to be inserted.
Being rectangular in cross-section, arch slots can accept appropriately-sized square and rectangular archwires as well as round archwires. It is the relationship between square and rectangular wire and rectangular arch slots that is of particular importance in orthodontics. When an archwire is formed from rectangular wire, for example, and provided the dimensions of its rectangular profile closely coincide with the dimensions of the receiving arch slot, torsional forces can be transferred from the archwire to the bracket when the two are engaged. In contrast, a round archwire is generally incapable of transferring torque in this manner.
Such torsional forces are ultimately transferred to the tooth root and the supporting bone, and allow the tooth to reposition over time. Correction of undesirably lingually or labially-inclined teeth is typically accomplished by corrective forces in terms of torque. Torque, along with other force vectors referred to as angulation, rotation, intrusion and extrusion, bodily movement and translation along an archwire are types of corrective forces that are transferred to teeth by an archwire. Torque in particular however requires the rectangular relationship between the archwire and its receiving slot in the bracket.
The mechanical relationship between a rectangular wire and a bracket's rectangular arch slot, and the orthodontic methods that employ that relationship are known as “Edgewise Mechanics”. The principles of Edgewise mechanics were introduced in the early 1900's but still play a central role in the practice of orthodontics today.
Over the course of treatment, orthodontic archwires must exhibit a wide range of mechanical properties to accommodate the changing force requirements of progressive phases of treatment. For example, teeth tend to be chaotically positioned and well out of alignment at the beginning of treatment. As orthodontic treatment begins, the arch slots of the brackets attached to these teeth will be similarly be chaotically-oriented relative to each other. An archwire ligated into such an array of mal-aligned arch slots must be capable of accommodating sharp bends, turns and twists without taking a set, while at the same time exerting gentle, safe, and continuous corrective forces to the teeth. In other words, archwires used at the beginning of treatment must exhibit a low spring rate at high deflection.
At a midpoint in treatment, the teeth and the arch slots attached to them will have responded to treatment by repositioning to a degree, thus bringing the arch slots of the brackets generally closer into alignment. The force requirements of an archwire at midtreatment are therefore different than at the beginning of treatment. The same general level of physiologically-corrective forces must be transferred to the teeth even though the deflection angles of the wire will have decreased. In other words, at mid-treatment, a wire exhibiting a moderate spring rate when moderately deflected is required.
At the end of treatment, the teeth, brackets and arch slots will fall much closer into a final alignment. In order to maintain the corrective force levels within a physiologically-effective range, such an archwire must have a very rapid spring rate at low deflections.
To accommodate this wide range of resilience needed over the span of treatment, orthodontists use archwires that vary in terms of physical dimensions, alloy and temper and even physical or metallurgical composition. An orthodontist may begin treatment with highly flexible archwires such as 0.012 or 0.014 in. round stainless steel with a relatively low tensile strength of about 125,000 PSI for example. Such wires may exhibit a modulus of elasticity (Young's modulus) of about 8,000,000 to 10,000,000. Woven wires consisting of multiple strands of smaller individual wires have also proven capable of handling the extreme contortions and twists encountered at the beginning of treatment, while delivering physiologically-appropriate corrective forces. During the finishing stage however, orthodontists use what are termed “finishing wires” that are formed from near spring-temper, work-hardened stainless steel or heat-treated cobalt-chromium alloys. Finishing wires typically exhibit a tensile strength approaching 300,000 PSI, with corresponding hardness. Such wires are provided in rectangular cross-sections to fully exploit Edgewise mechanics. The dimensions of such rectangular finishing wires completely fill the rectangular arch slot so that in addition to the material being harder and stiffer, there is more cross-sectional area of material contributing to force generation. In terms of modulus of elasticity (Young's modulus), very flexible archwires used at the beginning of treatment may exhibit a modulus as low as 8,000,000. Whereas at the other extreme, very rigid finishing wires may exhibit a modulus of over 31,000,000.
From the historical perspective, in the late 1800's, orthodontic wire was first formed from precious metals such as palladium, gold and platinum. Stainless steel was introduced to dentistry in the early 1930's. Due to the wide range to which stainless can be work hardened and its other desirable qualities, stainless steel has almost entirely replaced precious metals in dentistry.
In the early 1960's a remarkable alloy consisting of about 55% nickel and 45% titanium by weight was developed as a product of military research. Given the name Nitinol, it held great promise for meeting the long-sought orthodontic objective of achieving very light and gentle forces at very high deflection angles. Nitinol is in fact very gentle. In comparable shapes and in terms of its modulus of stiffness, Nitinol is only about 25% as stiff as equally-sized standard orthodontic stainless steel wire. Other unique properties of Nitinol involve its extraordinarily gentle spring rate. In particular, Nitinol demonstrates a unique loading plateau known as super elasticity. After moderate deflection, further deflection generates very little additional force. Nitinol's nearly flat super elastic stress/strain trait is maintained through a very wide range of deflection. Such properties are seen as ideal for delivering the gentle, continuous forces needed in orthodontics.
When first introduced to orthodontics, Nitinol quickly became appreciated as being perhaps the ultimate orthodontic wire because of its combination of remarkably desirable force characteristics. A refined version of the material was developed for orthodontic use and its very desirable properties provided the basis for successful commercialization. Today, Nitinol has been utilized in the fabrication of nearly every type of orthodontic energy-storing spring device. U.S. Pat. No. 4,037,324 to Andreason describes basic methodologies for integrating Nitinol into orthodontic treatment. A technical discussion of the unusual metallurgical properties of Nitinol is presented by Garrec et al., “Stiffness in Bending of a Superelastic Ni—Ti Orthodontic Wire as a Function of Cross-Sectional Dimension,” The Angle Orthodontist, vol. 74, no. 5, pp. 691-696 (2003), which is incorporated herein by reference.
Regardless of temper and metallurgical composition, all low spring-rate wires used in orthodontics, including Nitinol, share the property of being very flexible and at the same time, resistant to yielding (taking a set) unless very high deflection angles are attempted. Due to the extreme flexibility of these wires, orthodontists usually do not attempt to install activations or corrective adjustment bends in such wires. The highly flexible wires used early in treatment can have either a round or rectangular cross-section. Nitinol wires are commercially available in a rectangular cross-section, including full-sized slot-filling configurations. Using such wires allows practitioners to use such wires in an Edgewise mechanics-mode. This means that correction in terms of torque can be desirably pursued from the start of treatment. Again, very flexible early phase wires are usually relegated to initial unscrambling and leveling of a malocclusion, which involve relatively large movements separate from aesthetic positioning considerations later in treatment. Achieving torque objectives, even from the earliest point in treatment, has however proven advantageous.
Returning to the earlier discussion of Edgewise mechanics, a more detailed description of current and historical practice will serve to highlight the utility of the present invention. Today, commercially-available bracket systems consist of brackets where the orientation of the arch slot is optimized on a tooth-by-tooth basis to reflect the ideal aesthetic positioning of each tooth. In the past however, Edgewise mechanics was first employed using brackets that were all identical. No such tooth-by-tooth bio-engineered features were incorporated into bracket fabrication prior to the early 1970's. Early orthodontists were required to form highly detailed archwires in order to impart the corrective forces and orientations needed by each tooth. To do that, orthodontists used special pliers and torqueing wrenches to install specific types of bends in short segments of the archwire corresponding to the location of the bracket's arch slots. Those bends were referred to as first, second and third-order bends, and those bends are described below:
First-Order Bends. First-order bends involved a direct out-step or in-step of the archwire. Such a bend would establish the prominence of a tooth. An example of prominence can be seen in maxillary lateral incisors. Maxillary lateral incisors function best when inset slightly relative to the central incisors and the cuspids. In aesthetic terms, without the insetting of upper lateral incisors, a patient's smile may appear unnatural. Upper and lower cuspids however are properly positioned in terms of prominence when outset. To place a first order bend in an archwire, an orthodontist would form a sharp inward-stepping zig-zag bend followed by another bend in reverse, stepping back out to the pre-established natural curve of the archwire. Such an in-step bend may be required to either outset the cuspids or inset the lateral incisors.
Second-Order Bends. Second-order bends are best described by looking directly at the facial surface of a tooth's crown and imagining its center point. Second order bends involve clock-wise or counter clockwise rotation of the tooth about that point (i.e., rotation about a horizontal axis passing perpendicular to the enamel and through this center point of a tooth). It can be said that each tooth has a statistically normal angle in this sense, generally called angulation. To place a second order bend in a wire, an orthodontist would install an uphill or downhill cant, followed by a returning set of bends back to the wire's predetermined form.
Third-Order Bends. Third-order bends involve the installation of short torsional bends where the torsioned segment is intended to inter-work with the rectangular slot walls and floor to swing the root of the tooth inward or outward. This was discussed earlier as the central attribute of Edgewise mechanics. To install a third-order bend, an orthodontist would place torqueing wrenches in the area to be torqued and on an adjacent segment and install the bend through torsion. A returning bend at the other end of the slot-engaging segment would return the wire to a zero torque value and its predetermined shape.
Today, even with extensively bioengineered brackets, orthodontists often need to install first, second and third-order bends in order to compensate for some error in their positioning of brackets when the brackets were originally bonded to the teeth. Another reason to resort to such bends is to over-activate or further bias an archwire against stubborn teeth that are slow to respond to standard force levels.
Yet other reasons exist for returning to the practice of installing first, second or third-order bends. U.S. Pat. No. 5,080,584 (Karabin) and U.S. Pat. No. 6,036,489 (Brosius) both teach the use of low-rate, high-deflection orthodontic wire of the type typically used at the beginning of orthodontic treatment. In both patents however, the wire is disclosed as having been formed into a rectangular cross-section. In other words, in addition to the unscrambling and leveling functions envisioned for their low-rate wires, the intent of these inventors is to also begin correction in terms of torque from the start of treatment. Beginning efforts toward achieving torque objectives at such an early point in treatment can shorten the overall duration of treatment and reduce the tendency for relapse of the teeth post treatment. Rectangular, first-phase archwires disclosed by the Karabin and Brosius patents have become popular.
Again, both Karabin and Brosius contemplate low-rate wires. Brosius in particular considers the application of super-elastic Nitinol as an Edgewise energy-storing device. To place such a consideration into perspective, it can be said that indeed, since the early 1980s, Nitinol wire products have proven to be extremely successful in the orthodontic applications, but with one significant limitation. Nitinol wire, in its ideal super-elastic condition is extremely hard to bend. Placing bends in super-elastic Nitinol requires extreme over-bending in order to cause the wire to yield sufficiently to accept deformation. This difficulty in forming sharp bends in Nitinol-type wire has been lamented as the one shortfall of Ni—Ti wire in orthodontic applications. The present invention directly addresses this shortcoming. In the broader perspective, all types of first-phase, low-rate wires share the same attribute of requiring large degrees of over-bending in order to accept a sharp bend feature. Woven wire will not accept an enduring bend at all. To illustrate the need for over-bending, a bend of 90 degrees as typically sought for archwires in many treatment situations. This can require extreme over-bending to the extent that the wire can double back and contact itself. This sort of interference causes a fixturing problem that is hard to accommodate using standard dental pliers at chairside. Further, it requires the bending to occur in an out-of-axis manner, resulting in a slight helical distortion.
Prior Art Pliers. Traditional orthodontic wire-bending pliers became standardized in both configuration and methods of use prior to the advent of the titanium-based low-rate wire alloys commonly used today. As a group, they are not capable of accomplishing any significant degree of over-bending, in contrast to the present invention. Pliers designed in the past were able to adequately form sharp bends in stainless steel wires, but are generally incapable of forming sharp bends in fully elastic Ni—Ti wire for example.
Another problem is presented by interference between the beaks of the pliers. For example, consider the wire-forming surfaces 35 and 45 of the beaks 30, 40 of the pliers 20 in FIG. 1 that over-bend the wire by more than 90 degrees. As the beaks 30, 40 are closed, there comes a point where the two wire-forming surfaces 35, 45 have a proximity to each other that is less than the effective wire dimension or diameter. With a conventional pliers hinge, this would result in an interfering relationship between the two beaks 30, 40 with the wire 10 between these wire-forming surfaces 35, 45. Now, a small interference (e.g., 0.005 in. or less) may be accommodated given some play in the structure of the hinge components of the pliers, or by some shared lateral flexing of the beaks. The combined lateral flexing of the beaks required in the current example of 0.005 in. may well be tolerated, but some scuffing of the upper and lower faces of the wire may occur. The problems that are encountered as higher over-bend angles are incorporated into the beaks are that the degree of scuffing of the wire, and the amount of give or flexing required of the beaks becomes unworkably excessive. For example, excessive scuffing of the wire can actually shear-off material and significantly scar the wire. Such blemishes are to be avoided in orthodontic wire because they often lead to breakage. Outward flexing of the pliers' beaks can also lead to rapid wear of the hinge and excessive flexing of the beaks can lead to embrittlement and breakage.
Solution to the Problem. The present invention provides pliers to accommodate the high degree of spring-back encountered when attempting to form controlled bends in low-rate orthodontic wire. In particular, the pliers include a hinge and spring that allow a range of lateral motion between the beaks of the pliers (i.e., relative motion parallel to the axis of the hinge) sufficient to prevent interference between the wire-forming surfaces of the beaks.